Digital multimeters are multi-range test instruments capable of measuring voltage, current, resistance, and other parameters. A digital multimeter operates by converting the parameter to be measured into an analog voltage within a predetermined voltage range. This analog voltage is then converted to a digital number or word by means of an analog-to-digital converter. The digital word is then displayed on a digital display.
More particularly, an input voltage, or a voltage generated from an input current or an unknown resistor, is converted into an analog voltage within the input range of the analog-to-digital converter by applying the voltage to a resistive divider network. Generally, a separate resistive divider network or string is used for voltage, current and resistance measurements. Each resistive divider has multiple output taps and the divider is constructed in a known manner so that the input voltage is reduced at each tap by a predetermined ratio. Depending on the function (type of measurement) and range (of such measurement), one divider string is selected and an appropriate output tap is chosen to divide the input voltage down so that it lies within the input range of the analog-to-digital converter.
Many prior art digital multimeters utilize mechanical rotary switches for selecting the function and range for the measurement. These mechanical rotary switches have electromechanical contacts which connect one of the resistive divider strings to the appropriate input to make a measurement and connect the input of the analog-to-digital converter to one of the output taps from the resistive divider networks in order to select the appropriate measurement range. Such mechanical rotary switches are typically complicated and expensive to manufacture and somewhat awkward to operate. In addition, the mechanical contacts are subject to corrosion and dirt, either of which can render the meter inoperative.
Further, a mechanical rotary switch must often be turned "through" a measurement range to get to the desired range. For example, it may be necessary to turn the switch through the resistance measurement ranges to position the switch for a voltage measurement. During this process, the analog-to-digital converter can be damaged if the meter is left connected to the test input during the switching operation. Damage can also occur if a user selects an incorrect function and/or range.
This damage occurs because of the potentially harsh operating environment in which such meters are typically used. This operating environment may include measurement of voltages of a thousand volts or more and currents up to several amperes. In this operating environment, if a low voltage range is selected and a high voltage is actually applied to the meter, the resistive divider will not divide the voltage down sufficiently to place the reduced voltage within the operating range of the analog-to-digital converter and the over-voltage condition may be sufficient to damage the converter. This condition is exacerbated in most conventional meter designs because, in order to increase sensitivity at low measurement ranges, there is generally at least one meter setting in which the converter input is connected directly to the test input terminals. Such a connection renders the converter extremely vulnerable to over-voltage conditions.
In addition, conventional meter designs are generally not suitable for direct use with typical integrated circuits. Most integrated circuits operate with relatively low voltages on the order of a few volts. Since the meter must be able to operate with voltages on the order of a thousand volts, mechanical switch contacts, relay contacts or high-voltage semiconductor switches must be used to isolate the meter from the applied voltages. Electromechanical contacts have the disadvantages listed above and high-voltage semiconductor switches are expensive.
Another problem encountered with prior art digital multimeter circuits is that a single input terminal is used for both AC and DC voltage measurements. In order to avoid electrically loading a circuit which is being measured, the DC voltage measurement circuit is designed so that it has a high input impedance at the measurement input terminal. During an AC measurement, the high input impedance combines with stray capacitances at the meter input to cause rolloff of the AC voltage. As a result, prior art digital multimeters typically have poor high frequency AC sensitivity. A conventional prior art approach to solving this problem is to apply small trim capacitors across the resistors in the resistive divider network. These trim capacitors change the division ratio at high frequencies to compensate for the rolloff caused by the input impedance. Aside from increasing the manufacturing cost, these resistors create the need to use more complicated external switching, further increasing the overall meter cost.
Accordingly, a general object of the present invention is to provide a resistive divider network for a digital multimeter circuit which network uses a single divider string for all measurements, including voltage, current and resistance.
Another object of the present invention is to provide a resistive divider network in which the required function and range switching can be easily performed by low voltage, high performance, integrated circuit switches, thereby rendering the network amenable to complete integration.
Yet another object of the present invention is to provide a resistive divider network which includes high voltage input protection and thus allows the test inputs to be connected directly to the low voltage integrated circuit chip without costly high-voltage isolation switches.
A further object of the present invention is to provide a digital multimeter which separates the AC and DC voltage measurement paths at the input terminal, thereby eliminating the prior art rolloff problem.
Still a further object of the present invention is to provide a digital multimeter which can be microprocessor controlled.